1. Field of the Invention
The invention relates to a filter device for cleaning plastics (polymer) melts having a filter wheel for pivoting screening elements in and out of a melt channel through which the plastics melt flows in the filter device. The screening elements cover cavities arranged in the filter wheel and are exchangeable for fresh or cleaned screening elements at a removal/fitting opening of the filter device.
2. Description of the Related Art
German Patent Publication No. 42 12 928 A1 discloses a filter device where the individual screening elements include a number of filter cells arranged one above the other and which are operated parallel to one another. The filter cells are formed by two annular filter disks. The inner zones of the filter disks are connected to an interposed ring, which has radial passages, and the outer zones of the filter disks are connected to each other. The filter disks are formed by a multi-layer construction and have at least one filter layer and a supporting layer carrying the filter layer.
German Patent Publication No. 41 13 501 A1 relates to screening elements present in the openings of a rocker arm or a disk which, by pivoting or turning of the arm or disk, can be brought both into an operable position and into a removal position. Used elements can be replaced in the removal position by screening elements having new or cleaned filter areas.
A continuously operating filter system described in German Patent Publication No. 42 12 928 A1 employs a filter wheel through which a monomer melt flows eccentrically in a filter housing. The filter housing is fitted with screening elements or filter inserts which are arranged along a circle. The small pore widths of the filter inserts have a high cleaning effect, where the quality of the end product is improved. The filter wheel is equipped on the outer circumference with a toothed rim, which is engaged by a pinion. The pinion is connected via a lever to a drive piston, the movement of which is controlled by a hydraulic control system. The hydraulic device simultaneously serves for driving a backflushing piston (accelerator) for back-thrust cleaning of the filter cells with the screening elements and for driving a discharge piston. In the case of backflushing, the discharge piston channels the monomer to the outside and, in the case of normal operation, the discharge piston returns the monomer to the main melt line. The accelerator produces the required melt flow, briefly at a high rate, to backflush with the necessary pressure.
However, due to the high temperatures during operation, oxidation of the monomer or polymer of the plastics melt can easily occur during a filter change. Furthermore, it is not possible to blow a gas into the filter housing to reduce the risk of oxidation during the filter change, because cooling the filter wheel and screening elements could cause the plastics melt to solidify. This would inevitably lead to leaking or destruction of the filters. The high temperatures of the filter housing also have the effect of heating all the built-on parts so that their serviceability is no longer ensured. In this case, overloading of the drive elements, such as, for example, the drive piston, may also occur. The low mechanical support of the overall filter system is also problematical in the thrust-like backflow cleaning of the screening elements.